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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19
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NASA NTRS1982

Resonant electrodynamic heating of stellar coronal loops - An LRC circuit analog

The problem of electrodynamic coupling of stellar coronal loops where beta is less than 1 to underlying velocity fields where beta is greater than approximately 1 is treated. A rigorous analysis reveals that the physics can be represented by a simple yet equivalent LRC circuit analog. This derived analog suggests the existence of global structure oscillations which resonantly excite internal field line oscillations at a spatial resonance within the coronal loop. Even though the width of this spatial resonance, as well as the induced currents and coronal velocity field, within the resonance region explicitly depends on viscosity and resistivity, the resonant form of the generalized electrodynamic heating functions is virtually independent of irreversibilities. This is a classic feature of high-quality resonators that are driven externally by a broad-band source of spectral power.

Ionson, J. A.↗
NASA NTRS1982

The cooling and condensation of flare coronal plasma

A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The important features of this model which distinguish it from previous models of flare cooling are: (1) Most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation. (2) Flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints. (3) The differential emission measure has a strong temperature dependence. These results can account for recent observations of compact flare loops that are not consistent with the previous cooling models.

Antiochos, S. K.↗
NASA NTRS1982

Analysis of coronal H I Lyman alpha measurements from a rocket flight on 1979 April 13

It is noted that measurements of the profiles of resonantly scattered hydrogen Lyman-alpha coronal radiation have been used in determining hydrogen kinetic temperatures from 1.5 to 4 solar radii from sun center in a quiet region of the corona. Proton temperatures derived using the line widths decrease with height from 2.6 x 10 to the 6th K at 1.5 solar radii to 1.2 x 10 to the 6th K at 4 solar radii. These measurements, together with temperatures for lower heights determined from earlier Skylab and eclipse data, suggest that there is a maximum in the quiet coronal proton temperature at about 1.5 solar radii. Comparison of measured Lyman-alpha intensities with those calculated using a representative model for the radial variation of the coronal electron density yields information on the magnitude of the electron temperature gradient and suggests that the solar wind flow was subsonic for distances less than 4 solar radii.

Withbroe, G. L.↗
NASA NTRS1982

Energy balance of stellar coronae. I - Methods and examples. II - Effect of coronal heating

Simplified models of magnetically open coronal regions are computed, with the aim of fulfilling appropriate boundary conditions at the base of the atmosphere, at the critical point, and at infinity. The models are determined by the stellar mass and radius and by the amount and location of coronal heating, and this dependence is analyzed in terms of pressure, temperature, characteristic heights, energy losses, mass loss, and asymptotic behavior. The results are used to classify the magnetically open coronal regions according to the energy loss mechanism that dominates in the region between the base and the critical point, and it is shown that more complicated heating mechanisms may be replaced by the presented exponential heating law, provided that the damping length is suitably chosen.

Hammer, R.↗
NASA NTRS1983

An investigation of coronal active region loop structures using AS&E rocket X-ray images

Simultaneous high spatial resolution observations at 6 cm in soft X-rays, in photospheric magnetograms, and in optical filtergrams were used to compare the most intense sources of centimetric emission in two active regions to coronal loops, sunspots, chromospheric structures, and photospheric magnetic fields. Results show that the majority of the bright microwave components are not associated with sunspots or X-ray emission. A nonthermal mechanism appears necessary to explain the brightest microwave components, discrete regions of continuous particle acceleration may be common in active regions. Studies of the plasma parameters of selected loops imply that the radio emission is consistent with gyro-resonance absorption at the third and fourth harmonic, at least from part of each loop. Results are presented for: (1) X-ray and microwave observations of active regions; (2) comparison of coronal holes observed in soft X-rays and Hel 10830 A spectrosheliograms; and (3) the reappearance of polar coronal holes and the evolution of the solar magnetic field.

Webb, D. F.↗
NASA NTRS1982

Simultaneous radio scattering and white light observations of a coronal transient

Reports that a coronal transient observed by the Solwind coronagraph off the west limb of the sun was also seen in the spectral broadening observations of the Helois-2 2.3 GHz radio signal. These simultaneous data are compared and applied to studies of the shock front. UT difference images show a time difference of up to 2 h between shock and white light occurrences. Helios and Voyager shock speed measurements are consistent, implying that the event observed by Helios is a shock. These coronal transient data show that the shock front is ahead of the white light front, but additional observations of larger events will contribute information to the understanding of coronal transient evolution as they propagate away from the sun.

Woo, R.↗
NASA NTRS1983

The properties of coronal voids

Skylab coronagraph observations of the outer solar corona have revealed dark, ray-like structures whose identification as a true coronal phenomenon rather than a subtle photographic effect is presently undertaken. Photometric analysis indicates that these features represent a reduction of 5% in coronal radiance, and the use of a previously developed model of the coronal electron component allows the specification of void electron densities over a 2.5-4.5 solar radius range. Projection of the phenomena onto synoptic surface maps indicates a close relationship with filaments and neutral lines, and a potentially significant temporal correlation between void formation and the formation of the underlying prominence is noted.

Macqueen, R. M.↗
NASA NTRS1983

Coronal heating by waves

Alfven waves or Alfvenic surface waves carry enough energy into the corona to provide the coronal energy requirements. Coronal loop resonances are an appealing means by which large energy fluxes enter active region loops. The wave dissipation mechanism still needs to be elucidated, but a Kolmogoroff turbulent cascade is fully consistent with the heating requirements in coronal holes and active region loops.

Hollweg, J. V.↗
NASA NTRS1983

The effect of a coronal shock wave on the solar wind ionization state

In connection with studies of solar wind ionization state freezing, it is shown that, by using a Lagrangian approach of following individual fluid parcels, the techniques used previously for calculating ionization state variations in a steady state case can be extended straightforwardly to time-varying flows. The specific ionization state calculations presented are for a relatively simple picture of time-dependent coronal flow, based on a well-known model of a self-similar shock wave propagating through the corona. Time-dependent ionization effects for the sudden transition between two otherwise steady flows are likely to be limited to a narrow range of gas parcels which, having been shocked within the coronal freezing-in radius, pass a fixed interplanetary observer in an interval of a few tens of minutes. The amplitude of any rise in interplanetary ionization temperature associated with the coronal shock is likely to be considerably smaller than the jump in electron temperature that actually occurs in the corona.

Owocki, S. P.↗
NASA NTRS1984

Resonances of coronal loops

It is pointed out that any theoretical demonstration that the solar corona can be heated by waves requires a demonstration that the required energies can actually be carried from the convection zone to the corona by waves. In addition, it must be shown that the waves can dissipate their energy into heat in the corona. The present investigation is concerned specifically with the heating of closed magnetic structures in the corona, taking into account coronal loops or active region loops. Attention is given to the MHD Alfven wave. It is shown analytically that coronal active region loops can behave much like interference filters. The coronal part of the loop acts like a resonant cavity for Alfven waves. When the resonances are excited, large energy fluxes can be carried into the loop by Alfven waves which are generated in the solar convection zone. It is estimated that the energy fluxes can power the observed loops.

Hollweg, J. V.↗
NASA NTRS1985

Overheated Open Coronal Regions

The physics of compact, hydrostatic coronal shells formed from the collapse of overheated coronae is addressed. A large number of hydrostatic shells was computed for the usual exponential heating law with constant damping length. The boundary conditions were that on both sides of a shell the conductive flux at chromospheric temperatures is small. The boundary value problem was solved with a shooting technique. The main goal of the calculations was to determine the dependence of the shells on coronal heating flux (F sub Mo) and damping length (L); and in particular, to localize the boundary line in the parameter space (F sub Mo, L) that separates normal extended coronae from coronal shells.

Hammer, R.↗
NASA NTRS1985

Effects of coronal disturbances on the ionization state of the solar wind

A theoretical investigation of the relationship between physical conditions in coronal disturbances and the ionization states of the resulting perturbed solar wind was conducted. Available measurements of active solar wind associated with solar flares often show an enhanced degree of ionization (e.g., Fe(+17)), implying that the measured solar wind material is flare-heated at its coronal source. In contrast, other active solar wind flows sometimes show anomalously low degrees of ionization (e.g., He(+)), implying that some solar material is expelled from the corona without ever reaching coronal temperatures. The effects of impulsive heat and/or momentum addition in the corona on the hydrodynamics and ionization state of the corona and solar wind were studied. One of the primary objectives of the current grant was the development of a software package suitable for: (1) generating models for the effects of impulsive heat and/or momentum deposition on plasma temperatures, densities and flow velocities; and (2) calculating the ionization state of the solar wind as a function of the location, duration and magnitude of heat and/or momentum deposition.

Withbroe, G. L.↗
NASA NTRS1984

Characteristics of flares producing metric type II bursts and coronal mass ejections

An attempt is made to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60 percent of all flares with type II bursts for solar longitudes greater than 30 deg, where CMEs are well observed with the NRL Solwind coronagraph. H-alpha flare areas, 1-8 A X-ray fluxes, and impulsive 3-cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfven velocity.

Kahler, S.↗
NASA NTRS1985

A unified theory of coronal heating

Solar coronal heating mechanisms are analyzed within the framework of a unified theory of heating processes. The theory is based on the standing wave equation of Ionson (1982) for the global current driven by emfs from the convection Beta less than 1. The equation has the same form as a driven LRC equation in which the equivalent inductance is scaled with the coronal loop length. The theory is used to classify various heating mechanisms inside the coronal loops. It is shown that the total global current can be obtained from an integration of the local currents, the degree of coherency between local currents being the dominant factor governing the global current amplitude. Active region loops appear to be heated by electrodynamic coupling to p-mode oscillations in the convection Beta less than 1.

Ionson, J. A.↗
NASA NTRS1985

On the formation of coronal cavities

The formation of a coronal cavity and its relation to a quiescent prominence is studied theoretically. The stability of condensation modes of a plasma in the coronal streamer model (Steinolfson et al., 1982) is considered using a two-dimensional time-dependent ideal MHD numerical simulation. It is found that a plasma with beta = 0.5 is unstable but one with beta = 4 is stable because the density enhancement of the plasma trapped by the closed fields increases with the strength of the magnetic field. The means by which condensation modes can produce a coronal cavity and/or initiate the formation of a prominence (depending on the field configuration) are discussed. It is argued that prominence and cavity material is all supplied from the chromospheric level in the form of spicules.

An, C.-H.↗
NASA NTRS1986

Ephemeral active regions and coronal bright points: A solar maximum Mission 2 guest investigator study

A dominate association of coronal bright points (as seen in He wavelength 10830) was confirmed with the approach and subsequent disappearance of opposite polarity magnetic network. While coronal bright points do occur with ephemeral regions, this association is a factor of 2 to 4 less than with sites of disappearing magnetic flux. The intensity variations seen in He I wavelength 10830 are intermittent and often rapid, varying over the 3 minute time resolution of the data; their bright point counterparts in the C IV wavelength 1548 and 20 cm wavelength show similar, though not always coincident time variations. Ejecta are associated with about 1/3 of the dark points and are evident in the C IV and H alpha data. These results support the idea that the anti-correlation of X-ray bright points with the solar cycle can be explained by the correlation of these coronal emission structures with sites of cancelling flux, indicating that, in some cases, the process of magnetic flux removal results in the release of energy. That the intensity variations are rapid and variable suggests that this process works intermittently.

Harvey, K. L.↗
NASA NTRS1986

Coronal and Prominence Plasmas

Various aspects of solar prominences and the solar corona are discussed. The formation of prominences, prominence diagnostics and structure, prominence dissappearance, large scale coronal structure, coronal diagnostics, small scale coronal structure, and non-equilibrium/coronal heating are among the topics covered.

Poland, Arthur I.↗
NASA NTRS1986

Guided MHD waves as a coronal diagnostic tool

A description is provided of how fast magnetoacoustic waves are ducted along regions of low Alfven velocity (high density) in the corona, exhibiting a distinctive wave signature which may be used as a diagnostic probe of in situ coronal conditions (magnetic field strength, density inhomogeneity, etc.). Some observational knowledge of the start time of the impulsive wave source, possibly a flare, the start and end times of the generated wave event, and the frequency of the pulsations in that event permits a seismological deduction of the physical properties of the coronal medium in which the wave propagated. With good observations the theory offers a new means of probing the coronal atmosphere.

Roberts, B.↗